Answer:
You have 120 bottles of Cola and 120 bottles of Sprite.
You can exchange 3 empty Cola bottles for a new bottle of Sprite.
You can exchange 4 empty Sprite bottles for a new bottle of Cola.
You can borrow empty bottles, but must return them in the end.
What is the maximal number of bottles of drink that you can drink? Prove the optimality of your result.
As this is a puzzle, please give short and clever answers rather than tedious bruteforce calculations.